This invention relates to a method which is arranged to prevent the mixing of liquids in a riser of the kind utilized in the recovery of petroleum offshore. The invention also comprises a device for practicing the method.
In petroleum recovery offshore it is usual for a wellhead to be placed on the sea floor over the well opening in an early phase of the drilling work. The wellhead which is sealingly connected to the casing of the well, is provided with necessary blow-out preventers (BOPs) and connectors, for among other things a riser connecting the wellhead to a drilling vessel at the sea surface. In the drilling phase the well and riser are filled with drilling fluid.
A drill pipe/drill string which is provided with a drill bit at its lower end portion, is run from the drilling vessel through the riser, wellhead and further down into the well through the casing of the well to the bottom of the well, where the drilling takes place. Drilling fluid is circulated down the drill pipe to the drill bit, from where it flows, carrying cuttings, back to the drilling vessel, in the annulus between the drill pipe and casing/riser.
The riser is normally provided with several external smaller pipes (choke and kill pipes), which extend parallel to the riser and are connected at their upper end portions to processing equipment on the drilling vessel, whereas at their lower end portions they are connected to the wellhead at suitable points between the BOPs. The pipes may be used, for example, to replace the drilling fluid in the well if the well pressure increases in such a way that the riser, which normally has atmospheric pressure at the surface, must be shut off at the wellhead to prevent undesired outflow of drilling fluid.
In drilling it happens that bad weather, for example, makes disconnection of the drilling vessel from the well necessary. By such disconnection it is common that the part of the drill pipe located underneath the wellhead is hung off by means of an appropriate tool. The drill pipe portion located above the hanger tool is disconnected from it and pulled up. A valve in the wellhead is closed, thereby shutting off the lower portion of the drill pipe within the well. Thus, it is not necessary to pull up the entire drill pipe, which may take a long time.
After the lower portion of the drill pipe has been hung off and shut in within the well, the upper portion of the drill pipe is pulled up from the wellhead, as mentioned. However, before disconnecting the riser from the wellhead, the drilling fluid present in the riser must be replaced with water. The purpose of replacing the fluid is to take care of the well fluid and prevent it from contaminating the environment. Normally this is done by pumping water down to the wellhead through the external smaller pipes, so that the water displaces the drilling fluid out of the riser into the collecting tanks of the drilling vessel. Then the riser is disconnected from the wellhead.
It is a problem that in such replacing of fluid in the riser, in the area of contact of the two fluids, there is a considerable mixing of fluids. Drilling fluid thus becomes contaminated with water. Purification or destruction of such contaminated drilling fluid is relatively expensive.